New Computer Memory Confusion

My new Asus P6X58D-E motherboard manual (page 2-11) makes a statement that leads me to believe, if I use a certain memory, I will be limiting myself to be able to add more memory later.

The wording is : "Due to Intel spec. definition, X.M.P. DIMMS and DDR3-1600 are supported for one DIMM per channel only."

This says to me that if I populate sockets 1, 3, and 5 with say Corsair 1600 2 meg memory sticks for 6 megs, I would not be able to add more memory later using sockets 2, 4 and 6. But, if I used type 1300 sticks, I could use sockets 1, 3 and 5 now and later add more memory into sockets 2, 4 and 6.

Does this seem right to you?

Is there much penalty from not using the faster 1600 memory than the slower 1300 memory sticks?